1. Field of the Invention
This invention relates to the catalytic conversion of shale oil. More particularly, it relates to such conversion using a zeolite catalyst having a low constraint index and an outer shell having a lower SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio than the inner shell.
2. Discussion of the State of the Art
Catalytic reactions involving petroleum feed stocks have been known in the petroleum industry for a long time, e.g., the catalytic conversion of naphtha stocks. U.S. Pat. No. 4,191,638, for example, teaches most naphthas contain large amounts of naphthenes and aromatics and that, while the octane numbers are low, these stocks lend themselves well to catalytic conversion to gasoline stocks. On the other hand, shale oils, which have relatively high concentrations of paraffins and naphthenes, are not desirable as a feed to produce gasoline.
In U.S. Pat. No. 3,322,194, it is disclosed that vast quantities of hydrocarbons are contained in oil shale formations which are found in several parts of the world and particularly in the Piceance Creek Basin of the Green River Formation of Colorado. In these formations, the oil shale is not a true shale nor does it contain oil in the common usage of that term. The oil shale is a fine-grained, compact sedimentary rock which is generally highly laminated in the horizontal by bedding planes. It is more in the nature of marlstone. It contains an organic matter, kerogen, which is an amorphous organic solid. Kerogen, particularly, is defined as an organic, high molecular weight mineraloid of indefinite composition. The kerogen is not soluble in conventional solvents but will decompose by pyrolysis upon being heated to temperatures above 500.degree. F. to provide fluid hydrocarbons commonly termed "shale oil". Generally, the decomposition is undertaken at temperatures about 900.degree. F. However, excessive temperatures are usually avoided in the pyrolysis of kerogen to avoid heat consumption by the decomposition of the mineral carbonate constituents in the oil shale. Thus, oil shale must be heated in a process of pyrolysis, which process is usually termed "retorting", in order to obtain the desired recovery of hydrocarbons. For this purpose, it is necessary to either mine the oil shale and then retort it at the earth's surface, or to retort it in-place.
U.S. Pat. No. 4,112,056 discloses an improved process for making a zeolite having a high-silica-alumina mole ratio comprising adding a source of aluminum ions to a silica-rich amorphous reaction medium at a rate whereby the concentration of aluminum ions in the amorphous phase is maintained at a steady state.
U.S. Pat. No. 4,148,713 and U.S. Pat. No. 4,203,869 teach a crystalline aluminosilicate zeolite having an outer shell of silicate of the same crystal structure as the aluminosilicate.
Other patents applicants are aware of are U.S. Pat. Nos. 3,554,900; 3,615,188; 3,624,003; 3,625,866; 3,629,152; 4,050,539; 4,060,480; 4,088,605; 4,090,981; 4,241,036; and 4,246,138. It should be noted that they are of interest only in that although they all deal with catalysts and some deal with catalyst surface treatment, none teach or suggest the zeolites of this invention or their use or preparation in the manner claimed.